Thermoplastic resins such as polyethylene terephthalate (PET) are commonly used to manufacture packaging materials. PET processed under the right conditions produces high strength articles with excellent gas barrier properties. Foods, beverages, and medicines can deteriorate or spoil if exposed to oxygen. To improve shelf-life and flavor retention of products such as foods, beverages, and medicines, therefore, the barrier protection provided by PET is often supplemented with additional layers of packaging material or with the addition of oxygen scavengers.
Adding a layer of gas barrier film is known as passive-barrier packaging. Polyvinylidene dichloride (PVDC) is an example of a film commonly used for this purpose due to its excellent oxygen barrier properties. This method is not preferred, however, because it adds cost to packaging construction and does not reduce the levels of oxygen already present in the package.
Adding oxygen scavengers to the PET resin is known as active-barrier packaging. This approach to protecting oxygen-sensitive products is two-fold; the packaging prevents oxygen from reaching the product from the outside, and also absorbs any oxygen present in the container. In some applications, small packets or sachets containing oxygen scavengers are added to the packaging container. Iron powder is commonly used for oxygen scavenging in food packages. Iron reacts with the oxygen and forms iron oxide. Most applications also utilize a salt and a moisture absorber as reaction-enhancing agents to increase the effectiveness of the iron powder. Even when a salt and a moisture absorber are added, one difficulty with scavenger systems utilizing iron is the inefficiency of the oxidation reaction. High loadings of iron powder, on the order of 1000-2500 parts per million, are typically required to obtain sufficient oxygen absorption. Unfortunately, previous attempts at preparing resin compositions comprising high levels of iron have resulted in packaging materials with poor optical properties. Typically, bottles prepared from such resin compositions are dark in color or translucent. Haze values for these bottles are generally high, and clarity is lacking.
Thus, there remains a need for packaging materials having acceptable visual aspects and comprising oxygen scavenging resin compositions. This invention relates to an oxygen-scavenging resin composition having utility in packaging and other applications. More specifically, this invention relates to a film-forming, oxygen-scavenging polyester resin composition having low haze. The present invention further relates to a method for incorporating high levels of oxygen-scavenging particles into a film-forming polyester resin composition with low haze.
In general the present invention provides a resin composition comprising: a film-forming polyester; and an effective amount of oxygen-scavenging particles comprising at least one oxygen-scavenging element; wherein the particles have a particle size distribution such that particles of less than about 25 microns in size do not exceed a concentration defined by the formula
ppm=512.3xc3x97d 
wherein ppm is the approximate concentration of particles of less than about 25 microns in size in parts per million by weight, and d is the apparent density of the particles of less than about 25 microns in size in grams per cubic centimeter.
The present invention also includes a resin composition comprising: a film-forming polyester; and an effective amount of oxygen-scavenging iron particles, wherein the iron particles have a particle size distribution such that particles of less than about 25 microns in size do not exceed about 1250 parts per million by weight of the resin.
The present invention also includes a resin composition comprising a film-forming polyester and from about 50 to about 2500 parts by weight of iron particles per million parts by weight of the resin, wherein the concentration of iron particles of less than about 25 microns in size does not exceed about 1250 parts per million by weight of the resin.
The present invention also includes a polyester resin composition for use in forming transparent articles having low haze, the resin composition comprising from about 50 to about 2500 parts by weight of iron particles per million by weight of the resin, wherein said transparent articles have a Hunter haze value of about 10% or less.
The present invention also includes an article formed from a resin composition comprising an effective amount of oxygen-scavenging particles, wherein the Hunter haze value of the article is about 10% or less.
The present invention also includes a method for incorporating high levels of oxygen-scavenging particles into a film-forming polyester resin composition with low haze comprising the steps of: providing an effective amount of oxygen-scavenging particles comprising at least one oxygen-scavenging element, wherein the particles have a particle size distribution such that particles of less than about 25 microns in size do not exceed a concentration defined by the formula
ppm=512.3xc3x97d 
wherein ppm is the approximate concentration of particles of less than about 25 microns in size in parts per million by weight, and d is the apparent density of the particles of less than about 25 microns in size in grams per cubic centimeter; adding said oxygen-scavenging particles to a polyester resin composition during one or more of the process steps of: melt phase polymerization of the polyester; post polymerization and prior to pelletization; solid state polymerization of the polyester; and extrusion.
The present invention also includes a resin composition comprising: a film-forming polyester; and particulates; wherein the particulates have a particle size distribution such that particles of less than about 25 microns in size do not exceed a concentration defined by the formula
ppm=512.3xc3x97d 
wherein ppm is the approximate concentration of particles of less than about 25 microns in size in parts per million by weight, and d is the apparent density of the particles of less than about 25 microns in size in grams per cubic centimeter.
Advantageously, the present invention overcomes the problems associated with the prior art by providing a thermoplastic resin composition which contains an effective amount of iron or other oxygen scavenger and which has acceptable color and haze characteristics. The iron or other oxygen scavenger is present in an amount sufficient to effectively scavenge oxygen and provide longer shelf life for oxygen-sensitive materials. The particle size of the oxygen scavenger is optimized to provide effective scavenging activity, while reducing dark coloration and haze.